Japanese Unexamined Patent Publication No. 2004-197679 discloses a variable displacement compressor in which the opening degree of a displacement control valve provided in the housing of the compressor is controlled by determining whether an appropriate flow rate of refrigerant is produced in the refrigerant circuit. The opening degree of the displacement control valve may be varied by differential pressure acting on both sides of a restriction hole provided in a passage for discharging refrigerant. In this valve, differential pressure and electromagnetic force caused by supplying current to a solenoid in the valve counteract each other via the valve body. The opening degree of the valve is determined by the location of the valve body that is balanced by the counteraction.
In the above displacement control valve, the greater the flow rate of refrigerant in the refrigerant circuit, the greater the differential pressure on both sides of the restriction hole. As the differential pressure increases, the opening degree of the valve increases. When the flow rate of refrigerant exceeds an appropriate rate, the opening degree of the valve becomes larger and more refrigerant is supplied to a crank chamber from a discharge chamber through a valve hole. Thus, the increased pressure in the crank chamber decreases the inclination of a swash plate so that the flow rate of refrigerant is decreased to converge on an appropriate rate. When the flow rate of refrigerant is reduced to less than an appropriate rate, the opening degree of the valve becomes smaller and less refrigerant is supplied to the crank chamber from the discharge chamber through the valve hole. Thus, the decreased pressure in the crank chamber increases the inclination of the swash plate so that the flow rate of refrigerant is increased to converge on an appropriate rate.
When the compressor is provided with driving force from a vehicle engine, output control of the engine is necessary so that the engine output results in appropriate compressor torque. Since the flow rate of refrigerant reflects the torque of the compressor, the torque of the compressor can be estimated by detecting the flow rate of refrigerant. Although the differential pressure on both sides of the restriction hole reflects the flow rate of refrigerant, the flow rate of refrigerant is not directly detected. Accordingly, the flow rate of refrigerant, or the torque of the compressor, is estimated based on the magnitude of the current that is provided to the solenoid.
When the compressor is started, operation control to set the displacement of the compressor at 100% is performed. When the compressor starts working, liquid refrigerant that had remained in the crank chamber when the compressor stopped begins to evaporate. Then the pressure in the crank chamber is increased and the compressor continues working with the inclination angle of its swash plate remaining small. In other words, the flow rate of refrigerant is low. Meanwhile, the estimated flow rate of refrigerant based on the current supplied to the solenoid is greater. Accordingly, operation of the vehicle engine is controlled on the basis that compressor torque is great in spite of the fact that the actual torque of the compressor is small. This bring energy loss.
To address this, it is desirable to detect the flow rate of refrigerant in the variable displacement compressor by using a differential pressure-type flow rate sensor as disclosed in Japanese Unexamined Patent Publication No. 2004-12394. This sensor outputs an electrical signal based on differential pressure acting on both sides of a restriction hole provided in a passage for discharging refrigerant. The location of the sensor is not in the housing of the compressor but preferably in a passage forming member that forms a part of the passage for discharging refrigerant and that is detachably connected to the compressor housing. Thus, the sensor can be adjusted or corrected while the passage forming member is detached from the compressor housing. This makes adjustment and correction of the sensor easier than in a case where the sensor is located in the compressor housing.
The size of the restriction hole (or the cross-sectional area of the passage or the length of the hole) is an important factor for obtaining an appropriate differential pressure. However, when the restriction hole is provided in the housing or the passage forming member, to form a restriction hole having a desired size (or a desired cross-sectional area of the passage or a desired length of the hole) with high accuracy is difficult.